December 2004 - January 2005

Prospecting
for ice

By guiding a NASA plane into clouds that
harbor cold water drops, RAL researchers hope to improve
icing forecasts for the aviation industry.

Winter’s here and the time is right for flying into
ice.
These are early mornings for RAL’s Ben Bernstein, Frank
McDonough, and Cory Wolff. The trio is issuing predawn forecasts
from their Foothills Lab offices to help guide a NASA Twin
Otter turboprop airplane based in Cleveland into clouds with
a high icing potential. Their long-term goal: improve forecasts
of icing so pilots can be warned away from treacherous areas.

At times, they travel to Cleveland and ride on the plane,
comparing the accuracy of their forecasts with their own
observations of ice building up on the wings.

Most of the flying public would rather not head into hazardous
weather conditions. But, Cory says, “there’s
no better way to verify your forecast than to get on the
airplane and see what’s actually going on.”

The hunt for perfect ice

Protecting planes from in-flight icing is a high priority
for the aviation community. Ice formation on wings has been
a contributing factor in a number of fatal accidents, including
the 1994 crash of an American Eagle ATR-72 outside Chicago
that killed all 68 people on board. A 1997 icing-related
crash outside Detroit resulted in the deaths of the 29 people
on board.

Ice on the wing of the NASA Twin
Otter. (Photo courtesy NASA Glenn Research Center.)

But detecting treacherous regions in clouds that can produce
ice on aircraft wings remains a work in progress.

Icing occurs when supercooled water drops adhere to an aircraft
wing and freeze (supercooled drops are liquid even though
the temperature is below 32°F, or 0°C). When ice
builds up on the wings of an aircraft, it can simultaneously
slow velocity and decrease lift, potentially sending a plane
into a catastrophic dive.

The RAL team has collaborated with the NASA Glenn Research
Center in Cleveland every winter since 1997. The project
helps both agencies: RAL gets instant feedback on its forecasts
and obtains valuable information about the clouds from a
suite of on-board instruments, while NASA tests the performance
of its aircraft under actual icing conditions.

The challenge for the forecasters is to locate what one might
call the “Goldilocks” region, where conditions
are just right. A few degrees too warm, and the water drops
won’t freeze on the wings of an aircraft; a few degrees
too cold, and the water drops will turn into ice crystals
or snow instead.

The narrowness of that range came into focus on March 20,
2000, when United Airlines had to cancel numerous flights
because of severe in-flight icing conditions at Denver International
Airport. The icing conditions surprised meteorologists, who
had expected a heavy snowfall instead. Looking over data
after that event, the RAL team found the cloud tops that
day were warmer and lower than predicted. This meant that
supercooled water drops forming in the lower atmosphere were
not being cleared out by snow that would have formed if the
clouds had been deeper.

“There’s no absolute situation, no one type of
weather, no anything that tells you for certain there’s
icing,” Ben explains. “You just kind of have to
mix up this big soup of data and decide whether or not there’s
going to be ice.”

Over the years, the team has found that one perfect setup
for ice is a large, relatively uniform area of deep stratus
or stratocumulus clouds whose tops are a few degrees below
freezing (-12 to -5° C or 11 to 23°F). But subtle
factors can dramatically affect the severity of icing. Depending
on wind direction, for example, a 20-to-50-mile-wide (32-80
kilometer) band outside Cleveland often produces icing because
winds in that narrow area blow across both Lake Huron and
Lake Erie, creating a lake-effect icing cloud. This phenomenon
is like a lake-effect snowstorm, but it produces supercooled
liquid water instead
of snow.

To make the forecasts, the team members look at satellite
and radar data, and they weigh surface observations along
with pilot reports on ice. As Frank puts it, “We use
pretty much anything we can get our hands on that can give
us clues as to what’s going on in the atmosphere.”

Although forecasting icing conditions may be as much art
as science, the RAL team is getting good at it. The team
can guide the Twin Otter into icing conditions almost every
time. When the collaboration started, the success rate was
only
about 70%.

New tools for pilots

Thanks to the insights gleaned from the NASA collaboration,
RAL launched a new forecasting tool for airlines in 2002.
The Current Icing Potential (CIP) provides pilots with an
online display of high-precision maps that identifies areas
of potential aircraft icing produced by cloud drops, freezing
rain, and drizzle. It draws on surface observations, numerical
models, satellite and radar data, lightning observations,
and pilot reports. A companion product that is based solely
on numerical model output, the Forecast Icing Potential (FIP),
provides outlooks of icing conditions up to 12 hours in advance.

CIP and FIP have an 85% success rate in detecting icing conditions,
Ben says. But the tools often warn of icing conditions
when none exist, and the team wants to reduce
the number of false positives. Another goal is to provide
more detailed forecasts that will, in addition to alerting
pilots about the location of icing, also rate the severity
of the icing hazard.

The team also is putting a greater focus on a particular
type of supercooled drops, larger than 50 micrometers, which
can be especially dangerous. Small supercooled drops usually
freeze when they hit the front of an airplane wing, and many
planes are equipped with deicing devices designed to melt
them. Large supercooled drops, however, flow back over the
wing before freezing, or strike the wing rear of the deicing
equipment, creating crusty ridges of ice that can be destabilizing.

The Federal Aviation Administration is expected to issue
a rule that could require aircraft manufacturers to create
devices to protect wings from large-droplet icing. But designing
such devices may be difficult until more is known about the
behavior of large-droplet icing.

In two years, RAL will begin gathering even more data on
icing conditions. That’s because NASA will begin deploying
an S-3 Viking jet with the range to cruise over the entire
continental United States instead of being restricted to
a radius around Cleveland. This will allow the team to examine
regions such as the Pacific Northwest, where icing conditions
are believed to vary considerably from the Great Lakes.

“It will open up some exciting research possibilities,” Ben
says.

The CIP identifies areas of potential
icing. This graphic, from the CIP Web site, shows icing
at 15,000 feet.